Cathode ray tube



Oct. 31 1939.

A. J. YOUNG 2,177,762

CATHODE RAY.TUBE

Filed July 23, 1936 2'0 40 6'0 8'0 100 120 yous 6 7 5 a I L/ 20 4o so rams MENTOR 'ARTHUR J. YOUNG ATTORNEY Patented Oct. 31, 1939 CATHODE RAY TUBE Arthur James Young, London, England, assignor 1 to Radio Corporation of America, a corporation of Delaware Application July 23,

1936, Serial No. 92,054

In Great Britain August 31, 1935 2 Claims.

This invention relates to cathode ray tubes and more particularly to cathode ray tubes of the kind wherein the cathode ray is deflected electrostatically.

The invention is applicable alike to the common form of cathode ray tube wherein beam deflection in two co-ordinate directions is obtained electrostatically as regards both co-ordinate directions of deflection, and to cathode ray tubes wherein one co-ordinate direction of deflection is effected electrostatically and the other is effected electromagnetically.

In cathode ray tubes wherein deflection ofthe cathode ray is effected electrostatically, it is common practice to maintain one of a pair of electrostatic deflecting plates at a fixed potential and to vary the potential of the co-operating deflecting plate. With such an arrangement it is found, however, that the deflectional sensitivity when the deflecting plate whose potential is varied is made positive is not the same as when it is made negative. This isexemplified graphically in the accompanying Figure 1 in which actual deflection in millimeters (ordinates) is plotted against deflectingvoltages (abscissae). The curves were obtained by tests with a known ordinary high vacuum television receiver tube with mutually perpendicular pairs of deflector plates having 1000 Voltes on its second anode, curves I and 2 being the deflection-voltage curves for the pair of plates nearer the anode and curves 3 and 4 being the curves for the pair of plates further from the anode. For curves l and; the plate to which the deflecting voltage was applied was negative with respect to the anode while for curves 2 and 4 the actual deflecting plate was positive. For all curves the three plates other than that to which the deflecting voltage was,

applied were connected to the second anode of the tube. It will be observed that the diflerence of sensitivity is greater (for these tests) between curves 3 and 4 than between curves l and 2. The variation in deflectional sensitivity exemplified by Figure 1 gives rise to a well known form of distortion which in view of the effect produced may be termed trapezium distortion and while this effect may in some cases be reduced or even almost completely overcome by a push-pull excitation of the deflecting plates so that while one is made positive the other is made negative to a proportional extent, it .is obviously undesirable that cathode ray tubes should, when manufactured, be suitable for use only in these circumstances and even with push-pull excitation of the deflecting plates an appreciable amount of distortion may remain in' some cases. The present invention has for its object to provide an improved cathode raytube wherein trapezium distortion is substantially reduced or eliminated irrespective of the type of deflecting circuit arrangement employed.

Analysis of the cause of trapezium distortion appears to indicate that the diiference in deflectional sensitivity is due to distortion of the field at the edges of the deflector plates, the lines of force at the edges not being'perpendicular to the mean beam axis, with the result that the electrons constituting the beam are accelerated or retarded to a greater or less extent in the direction of the beam axis. The magnitude of this eiiect is governed to some extent by the ratio of the dimension of the deflector plates in the direction of the beam to the spacing between the deflector plates.

According to the invention the production of trapezium distortion is reducedor avoided by modifying the distribution of the field at the edges of the deflector plates by means of a guard ring or rings arranged about the path of the electron b'eam and maintained at a suitable fixed potential. By the term guard ring is intended any metallic member in the form of a closed figure, whether said figure be circular or not, and

indeed, in the at present preferred arrangement in accordance with the invention, the guard rings are in the form of open-ended rectangular boxes.

In one form of cathode ray tube in accordance with the present invention and illustrated (so far as is necessary to an understanding of the said invention) in the accompanying Figures 2 and 3, two mutually perpendicular pairs of electrostatic deflecting plates 5, 5a. and 6, 6a (the plate 6a 2 it is behind plate 6) are positioned in succession at different points along the axis of the tube, the deflecting plates of each pair being parallel over a portion of their length and then diverging over the remainder of their length in the direction of electron flow. I is the final accelerating anode of the electron gun, the remainder of which is not shown. Between the two pairs of deflecting plates is arranged a first guard ring 8 which is made from a piece of a flat metal strip bent into the form of a square open ended box,.the length in the axis direction of the box thus formed being about 0.15" and the length of the sides of the box in the transverse direction being substantially equal to the widest spacing between the first pair of deflecting plates. The guard ring is also shown in Figure does not appear in the drawing since in Figure 3 which is a view of the electrode system 5, 5a, 8, taken at right angles to the view of Figure 2. Thus two opposite edges of the guard ring 8 substantially coincide with the adjacent edges or" the first pair of deflecting plates 5, 5a, when regarded endwise of the tube. A second guard ring 9 of similar form is arranged adjacent those edges of the second pair 6, 6a of deflecting plates remote from the source of electrons and these two guard rings are connected together and to the final accelerating anode of the electron gun of the tube as shown in Figure 2. These connections may, if desired, be made during manufacture of the tube, inside the envelope thereof. If desired three guard rings might be providedz-one between the anode l and the plates 5, 5a, a second ring between the plates 5, 5a and the plates 6, 6a (ring 8) and a-third ring (ring 9) between the plates 6, 6a. and the screen.

With an arrangement as above described experiments have shown that trapezium distortion is reduced from a value of the order of some 540% to less than 2% when measured under static conditions. In fact, when a tube like that for which the curves of Figure 1 were obtained was modified in accordance with this invention to make it as shown in Figures 2 and 3 and the tests of Figure 1 were repeated, the curves I and 2 of Figure 1 merged substantially into a single curve l2 as shown in the accompanying Figure 4 and the curves 3 and 4 merged substantially into a single curve 34 as shown in the said Figure 4.

Although in the above described arrangement the inner faces of the guard rings are arranged to be parallel to the mechanical longitudinal axis of the electron gun assembly, this is not essential, nor is it essential that the guard rings be connected to the final accelerating anode. If desired, these rings may be maintained at different potentials from the said anode dependent upon the dimensions of the guard rings and upon the geometry of the deflecting plate system.

While the use of guard rings has been described and illustrated in connection with tubes having two pairs of deflecting plates it is to be understood that a single guard ring may be employed in cases where only one pair of electrostatic deflecting plates is employed. Again in tubes having more than two pairs of deflecting plates a suitably increased plurality of guard rings may be provided.

Having now particularly described and ascertained the nature of the said invention and in what manner the same is to be performed, I declare that what I claim is:

l. A cathode ray tube comprising means for developing a cathode ray beam, means for accelerating said beam, electrostatic deflecting means each comprising a pair of plates having a portion of their surface parallel with respect to and another portion of their surface at an acute angle with respect to the major aXis of said deflecting means, and anodic means having an aperture therein of a width at least as great as the greatest spacing between said plates at the ends nearest said anodic means, said latter means being located immediately adjacent said deflecting means, and a target for said beam.

2. A cathode ray tube comprising means for developing a cathode ray beam, means for accelerating said beam, electrostatic deflecting means each comprising a pair of plates having a portion of their surface parallel with respect to and another portion of their surface at an acute angle with respect to the major axis of said deflecting means, anodic means each comprising a narrow plane member bent so as to form a rectangular open box, said opening being of a width at least as great as the greatest spacing between said plates at the ends nearest said anodic means, said latter means being located immediately adjacent said deflecting means, and a target for said beam.

AR'IHUR JAMES YOUNG. 

